For the arrangement as shown in the figure, the magnetic induction at the centre is

• A

  $\frac{3{\mathrm{\mu }}_0\mathrm{i\pi }}{4\mathrm{a}}$

• B

  $\frac{{\mathrm{\mu }}_0\mathrm{i}}{4\mathrm{\pi a}}(1+\mathrm{\pi })$

• C

  $\frac{{\mathrm{\mu }}_0\mathrm{i}}{4\mathrm{\pi a}}$

• D

  $\frac{3{\mathrm{\mu }}_0\mathrm{i}}{8\mathrm{a}}$